Fix/prevent gcc 7.2 extra warnings

[mity]

Addresses #34

[codecov]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is 94.44%.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files           1        1           
  Lines        2526     2526           
=======================================
  Hits         2343     2343           
  Misses        183      183

Impacted Files	Coverage Δ
md4c/md4c.c	92.75% <94.44%> (ø)	⬆️

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[vtorri]

declare i as size_t instead of plenty of casts in 7d9cd9a

[mity]

Superseded by better #141